Rotatable connection systems that provide rotary unions for large structures, such as space station modules and connecting structures, impose severe demands on contact elements. In conventional roller bearing unions, the mechanisms that provide relative rotation, usually considered separate subsystems, rely on cables, bands, chains, or gears. These mechanisms are mechanically complex and have undesirable aspects such as large mass, sliding friction, difficulty of separation, and difficulty of electric signal transfer across the rotating union. Furthermore, the rolling elements experience high contact pressure and eventually wear out, notwithstanding the use of dry lubricants formulated for vacuum environments. Worn bearings cause rough operation and detrimental structural vibrations that must be controlled. Even the smallest structural vibrations unload some of the rolling elements, causing rattling. Rattling accelerates wear and induces high frequency acoustic noise in adjacent structures. Conventional rotary union systems also experience a problem referred to as slop. Slop generally encompasses phenomena such as gear backlash, bearing clearance play, elastic bending, mechanical compliance, and misalignment that all contribute to a failure of the rotary union to achieve a desired position, torque, or angular velocity. Also, distortion of large diameter bearing races leads to additional wear and the necessity for higher than desired rotating torque. The greater structural integrity needed to ensure bearing alignment usually results in an unacceptable weight penalty, particularly in space stations. In addition, differential thermal expansion between conventional steel bearing races and light metal or composite bearing support structures becomes a formidable problem in space, especially when one portion of the bearing is shadowed while the rest of the bearing is exposed to direct sunlight.
In operation, a rotary union system does not require much energy to maintain a constant angular velocity but it must have enough torque to accelerate or decelerate the joined structures at a desired angular rate and overcome increased friction as the mechanism ages. Electromagnetic motors, which have slip-rings and other commutating devices, may be used to drive the system, but motors are relatively inefficient and require heat removal in many applications. The sliding of motor brushes on annular slip-rings causes resistive heating, contact welding, electrical noise, and constant wear, all contributing to low system efficiency. Therefore, there is a need for an improved rotary union system for large structures that is relatively simple, light in weight, efficient, easily connected and disconnected with reduced slop, and able to transmit multiple electrical signals across the rotating connection.